Olefin polymerization catalyst systems can be classified into Ziegler-Natta and metallocene catalyst systems, and these two highly active catalyst systems have been developed in compliance with their characteristics. Ziegler-Natta catalyst has been widely applied to existing commercial processes since it was developed in the 1950's. However, since the Ziegler-Natta catalyst is a multi-active site catalyst in which a plurality of active sites are mixed, it has a feature that molecular weight distribution of polymers is broad. Also, since compositional distribution of comonomers is not uniform, there is a problem that it has a limitation to secure the desired physical properties.
Meanwhile, the metallocence catalyst comprises a combination of a main catalyst whose main component is a transition metal compound, and an organometallic compound cocatalyst whose main component is aluminium. Such a catalyst is a single-site catalyst which is a homogeneous complex catalyst, and offers a polymer having a narrow molecular weight distribution and an uniform composition distribution of comonomers, depending on the single site characteristics. The metallocence catalyst has characteristics which can control the stereoregularity, copolymerizing properties, molecular weight, crystallinity and the like of the resulting polymer by changing the ligand structure of the catalyst and the polymerization condition.
U.S. Pat. No. 5,914,289 discloses a method of controlling the molecular weight and the molecular weight distribution of polymers using metallocene catalysts which are respectively supported on supports. However, a large amount of solvent and a long period of time are required to prepare the supported catalysts, and the process of supporting metallocene catalysts on the respective supports is troublesome.
Korean Patent Application No. 10-2003-12308 discloses a method of controlling the molecular weight distribution of polymers by supporting a bi-nuclear metallocene catalyst and a mononuclear metallocene catalyst on a support with an activator, thus changing a combination of catalysts in a reactor and performing polymerization. However, this method has a limitation to achieve the properties of the respective catalysts simultaneously. In addition, there is a disadvantage that a metallocene catalyst portion is departed from a supported component of the catalyst to cause fouling in the reactor.
Therefore, in order to solve the above-mentioned disadvantages, there is a need to develop a method for preparing olefinic polymers with the desired physical properties by easily preparing a supported hybrid metallocene catalyst having an excellent activity.
On the other hand, a linear low density polyethylene is produced by copolymerizing ethylene and alpha olefin using a polymerization catalyst under low pressure. Thus, this is a resin having a narrow molecular weight distribution and a certain length of a short chain branch, without a long chain branch. The linear low density polyethylene film has high strength and elongation at break in addition to the properties of a general polyethylene and exhibits excellent tear strength, falling weight impact strength or the like. This has led to an increase in the use of a stretch film, an overlapping films or the like which is difficult to apply existing low density polyethylene or high density polyethylene.
However, the linear low density polyethylene using 1-butene or 1-hexene as a comonomer is mostly prepared in a single gas phase reactor or a single loop slurry reactor, and has a high productivity compared to a process using 1-octene comonomer. However, these products have limitations in the catalyst technology and process technology. Thus, they are problematic in that their physical properties are greatly inferior to when using 1-octene comonomer, and the molecular weight distribution is narrow, resulting in poor processability. Many studies have been conducted to improve these problems.
U.S. Pat. No. 4,935,474, describes a process for preparing polyethylene having a broad molecular weight distribution by using two or more metallocene compounds. U.S. Pat. No. 6,828,394 discloses a process for producing polyethylene which is excellent in processability and particularly suitable for a film, by using a catalyst system comprising a poor comonomer incorporating catalyst compound and a good comonomer incorporating catalyst compound. In addition, U.S. Pat. Nos. 6,841,631 and 6,894,128 describe that polyethylene having a bimodal or multimodal molecular weight distribution is produced by using a metallocene-type catalyst including at least two metal compounds, and thus it can be used in various applications such as film, blowing and molding, and pipe. However, although these products have improved processability, there is still a problem that the dispersed state per molecular weight within unit particles is not uniform, and thus the extruded appearance is rough even under relatively good extrusion condition and the physical properties are not stable.
Given the above circumstances, there is a constant demand for the production of superior products having a balance between physical properties and processability. In particular, the necessity of a polyethylene copolymer having excellent processability is further required.